A common all-reflective optical design used for imaging spectrometers is known as the Offner-Chrisp spectrometer, which is based on the Offner ring-field geometry. The classical Offner geometry is a 1:1 relay consisting of two concentric spherical mirrors, where the primary mirror is used twice in reflection. In an Offner spectrometer, the secondary mirror of the 1:1 relay is replaced with a reflective, convex grating having a groove spacing chosen based on the required spectral dispersion and physical dimensions of the imaging sensor.
The Offner-Chrisp imaging spectrometer, described in U.S. Pat. No. 5,880,834, for example, is a modified version of the Offner spectrometer, in which the primary mirror is split into two mirrors with slightly different radii. An example of the Offner-Chrisp optics geometry is shown in FIG. 1. The Offner-Chrisp spectrometer is a three mirror, ring-field design with 1:1 magnification where all three spherical mirrors are concentric, with a diffraction grating placed on the convex secondary mirror surface. Referring to FIG. 1, incident optical radiation 100 is received via an entrance slit (not shown), reflected from a first mirror 110 to the secondary mirror 120, where the diffraction grating disperses the optical radiation 100 into is spectral components to provide dispersed optical radiation 140 that is reflected from the secondary mirror 120 to a third mirror 130. The dispersed optical radiation 140 is reflected from the third mirror 130 and focused onto an imaging sensor 150. The Offner-Chrisp imaging spectrometer retains the near-concentric property of the classical Offner relay, while providing additional degrees of freedom toward the correction of aberrations by allowing the radii of curvature of the first and third mirrors to be slightly different. The radius of curvature of the diffraction grating, and therefore of the secondary mirror 120, is governed by the grating density and the spectral dispersion at the image plane (where the imaging sensor 150 is located), and may be determined by specifications such as the spatial and spectral image size needed for a particular applications. Since all three optical surfaces are concentric, the size of the spectrometer can be estimated based on the radii of curvature of the three mirrors and the distances between the secondary mirror 120 and first and third mirrors 110, 130. The optical design form of the Offner-Chrisp spectrometer is corrected for spherical aberration because the parent mirrors operate near their common center of curvature, and is corrected for coma by symmetry about the aperture stop. In addition, the Offner-Chrisp spectrometer is corrected for astigmatism when operating within the ring-field zone where third- and fifth-order astigmatism balance, but only for a single wavelength, which is typically chosen to be the central wavelength of the spectral bandwidth.
Although the Offner-Chrisp imaging spectrometer is well known, the optical form has significant performance limitations that reduce its suitability or usability in systems with large slit formats. The double-pass Reflective Triplet spectrometer optics configuration can handle larger slit formats and offers additional advantages such as multiple channel operation using multiple dispersive elements and multiple FPAs. However, the Reflective Triplet is more complex and costly than desired if only a single spectrometer channel is needed.
Some attempts have been made to achieve spectral broadening (increased spectral bandwidth) and spatial or field of view broadening (increased slit length) in the Offner-Chrisp spectrometer form using so-called “free-form” or Zernike-described optical surfaces, as discussed for example in Freeform spectrometer enabling increased compactness, Jacob Riemers et al., Light: Science & Applications (2019), 6, e17026; doi: 10.1038/Isa.2017.26 (hereinafter referred to as “Riemers”). However, while indeed achieving some improvements over the classical Offner-Chrisp geometry in certain respects, the designs in Riemers still have performance limitations and therefore disadvantages for some applications.